The rapid growths of data-centric services such as Internet video and social media are changing our lives and the way society communicates. Today with the initial expansion of these services and their requirement for general deployment of broadband access networks they are already driving the upgrade of dense wavelength division multiplexing (DWDM) networks from 10 Gb/s per channel to more spectrally-efficient 40 Gb/s or 100 Gb/s. With the anticipated growth of digital media globally, approximately $2.2 trillion over the next five years (see for example “Ultrafast Networks Gear Up for Deployment” (Nat. Photonics., Vol. 4, pp. 144), today's demand for broadband connectivity is driven by necessity, not luxury. To achieve sustainable economical growth around the world it is important to make sure the cost of the Internet is as low as possible, both in price and energy. This requires the development of a low cost energy efficient Terabit per second (Tbps) communication infrastructure with only optical fiber technology able to handle multi-Terabits per second of data effectively. Hence, an increase in data rates to 400 Gbps-1 Tbps in next-generation optical communication systems has become inevitable. In such systems, data integrity is stressed by physical layer impairments, i.e. chromatic dispersion, polarization mode dispersion and nonlinearity in the optical fiber. Advanced forward error-correction (FEC) technologies in optical links offer one of the most cost-effective methods to combat system impairments, increase the data rate, and extend its reach. Accordingly, techniques and technologies that enable the development of power efficient FEC decoders become important and beneficial.
Current optical networks employ forward error-correction (FEC) based on classical error-correcting codes such as Reed-Solomon (RS) or Bose-Chaudhuri Hocquenghem (BCH) codes, see for example Tychopoulos et al “FEC in Optical Communications—A tutorial overview on the evolution of architectures and the future prospects of outband and inband FEC for optical communications” (IEEE Circuits and Devices Mag., Vol. 22, No. 6, pp. 79-86) and ITU-T Recommendation G.975.1, “Series G: Transmission Systems and Media, Digital Systems and Networks, Digital sections and digital line system—Optical fibre submarine cable systems: Forward error correction for high bit-rate DWDM submarine systems” (ITU 2006). Both RS and BCH codes currently use hard-decision based receivers that have limited coding gain.
Since the re-discovery of iteratively decodable error-correcting codes performing very close to the Shannon capacity limit, wireless communications has been realigned to use these new powerful classes of codes including turbo codes and low-density parity-check (LDPC) codes see for example Gallager “Low-density parity-check codes” (IRE Trans. on Inf. Theory, Vol. 8, No. 1, pp. 21-28), Gallager “Low density parity check codes” (Ph.D. Dissertation, MIT Press, Cambridge, Mass., 1963), Berrou et al “Near Shannon Limit Error-Correcting Coding and Decoding: Turbo-Codes” (Proc. of IEEE Intl. Conf. on Comm., ICC 1993, Vol. 2, pp. 1064-1070), and Chung et al “On the design of low-density parity-check codes within 0.0045 dB of the Shannon limit” (IEEE Comm. Lett., Vol. 5, No. 2, pp. 58-60).
Although LDPC codes were invented by Gallager in 1962, they were widely overlooked until the 1990s, see Berrou, Chung, Tanner “A recursive approach to low complexity codes” (IEEE Trans. Inform. Theory, Vol. 27, No. 5, pp. 533-547) and Zyavlov et al “Estimation of the error-correction complexity of Gallager low density codes” (Probl. Pered. Inform., Vol. 11, No. 1, pp. 23-26). With the recent substantial increases in computing power, LDPC codes have generated great interest in the wireless community. Following this new paradigm, LDPC codes which use soft-decisions have been proposed for optical communication systems to mitigate the challenging optical channel impairments in the next-generation optical communication systems, see Vasic et al “Low-Density Parity Check Codes for Long Haul Optical Communications Systems” (IEEE Photon Tech. Lett., Vol. 14, No. 8, pp. 1208-1210) and Djordjevic et al “Low-Density Parity-Check Codes for 40-Gb/s Optical Transmission System” (IEEE J. of Sel. Topics in Quant. Electron., Vol. 12, No. 4, pp. 555-562, hereinafter Djordjevic1). The performance of different classes of LDPC codes has been assessed extensively through simulation taking into account certain major transmission impairments such as inter-channel and intra-channel nonlinearities, stimulated Raman scattering, group-velocity dispersion, optical amplifier noise, filtering effect and channel cross-talk, see Djordjevic. Results showed that LDPC codes can be an extremely effective solution for high speed optical systems achieving coding gain of as high as 11 dB and 5 dB over an uncoded and RS(255,239) based optical systems respectively, see Djordjevic et al “Next Generation FEC for High-Capacity Communication in Optical Transport Networks (Invited)” (IEEE J. of Lightwave Tech., Vol. 27, No. 16, pp. 3518-3530, hereinafter Djordjevic2).
LDPC codes can be very powerful, but their practical implementation for high data rate optical communications remains a challenge due to the complex structure of the decoder. Recently Tehrani et al “Relaxation dynamics in stochastic iterative decoders” (IEEE Trans. Signal Process., Vol. 58, No. 11, pp. 5955-5961) and Mohsenin et al “A low-complexity message passing algorithm for reduced routing congestion in LDPC decoders” (IEEE Trans. Circuits Syst. I, Reg. Papers, Vol. 57, No. 5, pp. 1048-1061) have reported work considering simplified decoder structures to enable the same performance. Most prior art and current research activities related to LDPC algorithms for optical communication are mostly focused on simulation studies with a few experimental demonstrations. Examples of such prior art include Mizuochi et al “Experimental demonstration of concatenated LDPC and RS codes by FPGAs emulation” (IEEE Photon Tech. Lett., Vol. 21, No. 18, pp. 1302-1304); Kobayashi et al “Soft-decision LSI operating at 32 Gsample/s for LDPC FEC-based optical transmission systems” (Proc. OFC and NFOEC, pp. 1-3, Paper OWE2, 2009); Onohara et al “Soft-decision FEC for 100G transport systems,” (Proc. OFC and NFOEC, pp. 1-3, Paper OThL1, 2010); Miyata et al “A Triple-Concatenated FEC Using Soft-Decision Decoding for 100 Gb/s Optical Transmission” (Proc. OFC and NFOEC, pp. 1-3, Paper OThL3, 2010); Masalkina et al “Soft-FEC Implementation for High-Speed Coherent Optical OFDM Systems” (Proc. 2010 ITG Symposium on Photonic Networks, Paper ITG-FB 222); and Yang et al in “428-Gb/s single-channel coherent optical OFDM transmission over 960-km SSMF with constellation expansion and LDPC coding” (Opt. Express, Vol. 18, No. 16, pp. 16883-16889). In Mizuochi, Kobayashi, Onohara, and Miyata a novel soft-decision all electrical front-end for a receiver was demonstrated where the soft-decision bits are provided by a 3-to-2 encoder. However, the soft-decision circuit consumed alone 14 W at 32 GS/s.
Amongst the challenges is that the decoding of the code requires soft-decision bits or multiple level of information about the received bits. Conventionally, optical communication systems rely on a hard-decision approach where the information about the received bits consist of only one bit, either a digital “1” or “0” was received. On the other hand, a 2-bit soft decision decoder requires 22−1=3 decision levels where the middle level is a bit corresponding to the hard-decision digit and the other two levels indicate the probability or confidence regarding the hard-decision, e.g. the received bit is certainly a “0” or maybe a “0” based on noise level or other physical layer impairments. As demonstrated by Kobayashi and Onohara the 32 GS/s 2-bit soft-decision circuit for LDPC decoders yielded a net coding gain (NCG) of 9.3 dB achieved at 126.4 Gb/s by combining four soft-decision circuits.
Recently, the inventors in Sakib et al in “Optical Front-End for Soft-Decision LDPC Codes in Optical Communications” (J. Opt. Comm. Net., Vol. 3. pp. 533-541, hereinafter Sakib1) have demonstrated a low complexity and energy efficient optical front-end for soft-decision decoder. The proposed front-end operating at 12.5 Gb/s consumed 5 W of power with NCGs of 2.75 and 6.73 dB at BERs of 10−4 and 10−9 respectively. Within this specification this concept as well as its extension to a 45 Gb/s optical front-end is presented. Optical receiver design according to embodiments of the invention is implemented by tapping the incoming optical signal prior to the photodetector and using an exclusive-nor (XNOR) gate. The receiver architecture is simplified by using a passive optical power divider instead of active electrical fan-out buffers and it is also shown that the soft-decision front-end can be used as a 2-bit flash analog-to-digital converter (ADC) for use with digital equalizers, for example, also making the receiver design suitable for systems requiring digital post-processing. For singlemode erbium doped fiber amplifier (EDFA) based optical systems, the noise distribution is Chi-Squared and symmetric Gaussian for direct detection and coherent receivers, respectively. However, within the context of multimode links the inventors refer to the concept of “flash ADC” for receivers as multimode links can be modeled as systems with additive Gaussian noise and symmetric Gaussian amplitude distribution such that the received optical signal can be equalized with digital equalizers. A flash ADC is a simple form of an ADC in which the input voltage to the ADC is divided into multiple levels to several comparators or limiting amplifiers enabling higher speed by parallelizing the digitization process. The outputs of these comparators are fed to a binary encoder that gives the digitized bits. Accordingly, the inventors have established that a soft-decision front-end may essentially perform the desired a flash ADC.
It would be as discussed supra be beneficial to reduce the power consumption of optical receivers for optical links exploiting LPDC encoding. Accordingly, the inventors have established a low complexity soft-decision front-end which is compatible with deployable LDPC codes in next-generation optical transmission systems. Beneficially the optical receiver design can be retro-fitted into deployed hard-decision based optical systems through the additional of a passive optical tap prior to the photodetector allowing the soft-decision front-end to be added in parallel thereby minimizing the additions to the optical infrastructure. Additionally the invention replaces the 3-to-2 encoder of the prior art in the electrical portion of the receiver with a single gate design. With the lowest cost short optical links operating at multi-Gigabit rates being multimode based it would be further beneficial for receivers in such links to exploit flash ADC techniques for improved performance.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.